I work with corrugated stock (you'd call it cardboard) a lot, and am doing so right now...or rather I should be
I saw this video many days ago since my friends know I'm big on both bikes (fun) and E-flute (not fun) so they figured this was right up my alley.
I have to say he's done some impressive things with kraftpaper honeycomb panels (this is like corrugated stock on steroids, not your regular cardboard pizza box) but some of his design choices are really weird: since you're using a bunch of manufactured components already, does anybody really want to ride on that "saddle"? Those "handlebars"?
You can source KHP that already have resin-impregnated liners (the outer sheets) and can even get them with the inner comb made waterproof with resin...it all adds both cost and weight and negates some of the "green-ness" of using a renewable and/or recycled material. But then this whole thing was sprayed heavily with paint (maybe epoxy) to give it at least some water resistance...not very green in practice or easily recycled in final form.
Sure this might represent only $10 worth of raw material just for the board, but once you tool-up for the die-cutting and add in all the off-the-shelf parts and the manufacturing, assembly and coating(s), it will go way up.